Tens of thousands of British citizens born in the US but who left when only a few months or years old risk having their bank accounts in Britain frozen because of intense pressure by US tax authorities on UK banks.

In one case, a 74-year-old living in Cambridge has been sent increasingly urgent letters from Barclays demanding her American tax identification number, even though she left the US on the RMS Queen Elizabeth in 1947 when she was just 18 months old. She had assumed that her US citizenship had lapsed.

The pensioner, who did not want her name published, said: “I felt hounded … I didn’t understand US tax law.”

An increasing number of Britons – many of whom have never spent a day of their working lives in the US – are being chased by their banks, which are insisting that they hand over their American tax identification numbers or risk having their assets frozen.

British banks are terrified of huge fines from US regulators if they continue to serve US citizens but fail to share information with the US Internal Revenue Service, the country’s taxation authority.

But this leaves thousands of so-called accidental Americans, who were born in the US but left as toddlers, in a near-impossible situation. Those born before 1986 were never allocated a tax identification or social security number or warned that they would be liable for US taxes for the rest of their lives.

The US is the only country in the world besides Eritrea that taxes non-resident citizens on their global income. The 2010 Foreign Account Tax Compliance Act (Fatca) requires foreign financial firms with US operations – including UK banks – to report information about US taxpayers to the Internal Revenue Service via HMRC.